1. Field of the Invention
This invention relates to exhaust treatment systems, and more particularly to efficient packaging and use of those systems.
2. Description of the Related Art
Stricter environmental regulations in the United States and Europe are requiring the use of exhaust treatment devices such as particulate filters and/or catalytic devices on diesel engines to reduce harmful emissions including nitrogen oxides (NOx), hydrocarbons, carbon monoxide, soot, ash, and other substances. Space is usually at a premium on diesel engines and their related components, particularly on trucks or other moving vehicles, and it is desirable that the additional exhaust treatment components be compacted into as small a space as possible.
The problem of efficient use of exhaust stream additive chemicals makes the issue of space more difficult, since many additives, after being injected or “dosed” into the exhaust stream, need to mix, undergo chemical change, be evaporated, or otherwise be modified from their original injected form before encountering exhaust treatment components such as flow-through catalytic devices or particulate filters.
For example, a particulate filter needs to be regenerated on occasion, regeneration meaning the removal of soot and other substances that have accumulated on the filter, by heating the filter to a high temperature and removing the soot through oxidation. In many cases, to reach the high temperatures needed for regeneration, a hydrocarbon (typically diesel fuel in the case of diesel engines) is dosed into the exhaust stream and allowed to mix with the exhaust gases and evaporate from its dosed liquid form. The hydrocarbon is then oxidized on an oxidation catalyst, releasing heat which raises the exhaust gas temperature to the level required for filter regeneration. Similar dosing of urea or ammonia is also often used as a reductant for of selective catalytic reduction (SCR) devices, which remove NOx from the exhaust.
For proper mixing and evaporation of the hydrocarbon it is necessary to provide a length of straight, open exhaust tube, more than 0.3 meters in many cases, before reaching the oxidation catalyst. If there are bends in the tube the hydrocarbon hits the side wall of the tube before evaporating and collects there. Too short a distance can result in unevaporated hydrocarbon mixing with soot in the exhaust system and gumming up the oxidation catalyst.
SCR devices are generally used in conjunction with upstream doses of urea. Typically, the urea is dosed into the exhaust stream, its water evaporates leaving urea crystals, and the increasing temperature of the crystals causes them to convert to ammonia, which mixes with the exhaust and is then used on the SCR to convert NOx to nitrogen and water. The length of tubing needed for the urea drying and decomposition process, as well as the ammonia mixing, may range from 0.3 meters to a meter, the tubing needing to be straight and free from section changes to avoid unevaporated urea from collecting on the side walls, a factor that also impacts other additives including hydrocarbon. In addition to reducing the amount of ammonia reaching the SCR device, the unevaporated urea can produce acids that corrode the tube material.
Other exhaust treatment devices that call for dosing include lean NOx catalysts and others. The space requirement of these additive treatments is difficult to incorporate into the limited space allocated to exhaust treatment.